Optical element molding die and method for molding optical element

ABSTRACT

An optical element molding die is designed for molding an optical element having a concave-convex structure. The optical element can be manufactured by a wet system that enables element formation over a large area and a curved surface, without using a lithographic process, and is advantageous in terms of mass production and equipment cost. The optical element molding die includes a substrate having a surface with a negative standard electrode potential in the oxidation reaction and an anodic oxidation layer provided on the substrate. A protective layer with the positive standard electrode potential is provided between the substrate and the anodic oxidation layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an optical element molding die for molding anoptical element having a concave-convex structure and to the opticalelement.

2. Description of the Related Art

Optical films with different refractive indexes, such as antireflectivefilms, have been provided individually or in a plurality thereof to athickness of from several tens to several hundreds of nanometers on thesurface of optical elements, thereby making it possible to obtain thedesired optical properties. Vacuum film formation methods such as vacuumdeposition and sputtering or wet film formation methods such as dipcoating and spin coating are used to form these optical films on thesurface of optical elements.

Optical elements called SWS (Sub-Wavelength Structure) that have amicroperiodic structure have been actively studied in recent years asoptical elements having the desired optical properties. Antireflectionfunction is known as a specific feature of optical elements having amicroperiodic structure. The antireflective function is realized byproviding a periodic structure with a period less than an incidentwavelength on a substrate. In recent years, the advancements inmicroprocessing technology made it possible to form extremely fine andcomplex patterns.

For example, such patterns are fabricated in a semiconductor processcentered on photoluminescence. In this method, a photoresist is coatedon a substrate that is used to form a concave-convex structure, exposureand development are conducted via the photomask, a resist mask patternis obtained, and the mask pattern is transferred by etching onto thesubstrate for forming the concave-convex structure. Further, a largenumber of researches have also been conducted to attempt the realizationof a concave-convex structure on the base of a naturally formed regularstructure, that is, a structure that is formed in a self-regulatedmanner. For example, a method in which an optical element having aconcave-convex structure is manufactured at a low cost by arrangingmicroparticles has been suggested.

An anodic oxidation method is also known as a method by which aconcave-convex structure can be formed over a large area at a low cost,and the aspect ratio can be randomly controlled. With this method,microholes are formed by using a metal such as aluminum as an anode inan oxidizing electrolytic solution, passing an electric currenttherethrough, and causing oxidation. A procedure using this method toarrange regularly the holes side by side has been developed. Forexample, a method has been developed for producing an optical elementmolding die by forming an Al film by sputtering on a die having apredetermined shape and then forming holes by anodic oxidation andobtaining a concave-convex structure, as described in U.S. Pat. No.7,268,948. This process is effective for providing a concave-convexstructure, while maintaining a highly accurate surface shape of a lensor the like.

A method for manufacturing an optical element molding die by using theconventional anodic oxidation method is a manufacturing method thateffectively makes it possible to form a concave-convex structure over alarge surface at a low cost and control randomly the aspect ratio. Inparticular, as described in U.S. Pat. No. 7,268,948, a method iseffective in which an optical element molding die is fabricated byforming an Al film by sputtering on a die a predetermined shape and thenforming a concave-convex structure by anodic oxidation. For the diesthat are used for high-precision molding of lenses or the like, a Niprocessed layer has been used most often due to good processability andstability in molding, and a concave-convex structure produced by anodicoxidation can be formed on the surface, while maintaining the surfaceaccuracy of the die, by forming an Al film on the processed Ni surfaceand conducting anodic oxidation. However, dust that is generated duringprocessing or dust from the atmosphere adheres to the processed Nilayer. The amount of this dust can be reduced by cleaning afterprocessing, but the dust is difficult to remove completely. As a result,the Al film is formed on the Ni layer on which the adhered dust ispresent. Ni has a negative standard electrode potential in the oxidationreaction, and when anodic oxidation is conducted, nickel is subjected toanodic electrolysis in the electrolytic solution. The dust that hasadhered to the Ni layer is also electrolyzed.

Where the dust is dissolved by the electrolysis or oxygen is generated,the oxidation state of Al differs from the usual oxidation state. As aresult, the desired microshape cannot be obtained. Another defect isthat external appearance changes locally due to variation in color tonecaused by the concave-convex structure in this portion. Further, anodicoxidation of Al proceeds by oxidation of Al in the dissolution process,but where the dust is present, dissolution also proceeds from the Alside surface of the boundary portion of the dust and Al. As a result,the electrolytic solution reaches the Ni layer, causing dissolution andgeneration of gas. As a result, the dust falls off, pinholes areproduced, and the desired microshape is difficult to produce. Further,because the Ni layer is subjected to anodic electrlysis, swelling iscaused by generation of oxygen or spots are produced by dissolution ofthe Ni layer and problems are associated with durability of the die.

SUMMARY OF THE INVENTION

The invention has been created with the foregoing in view. An aspect ofthe invention provides an optical element molding die that excels indurability and has a concave-convex structure formed to a high accuracyand also provides a method for manufacturing an optical element moldingdie. Another aspect of the invention provides an optical element thathas a function of inhibiting an interface reflection light amount at thelight incoming-outgoing surfaces and a method for molding the opticalelement.

In order to attain the above-described aspects, the optical elementmolding die in accordance with aspects of the invention includes asubstrate having at least a surface composed of a material with anegative standard electrode potential, an anodic oxidation layerprovided on the substrate, and a protective layer composed of a materialwith the positive standard electrode potential between the substrate andthe anodic oxidation layer.

The method for manufacturing an optical element molding die inaccordance with aspects of the invention includes the steps of forming aprotective layer comprising a material with a positive standardelectrode potential on a substrate that has at least a surface composedof a material with the negative standard electrode potential; forming analuminum layer on the protective layer; and forming a plurality of holesin the aluminum layer and forming an anodic oxidation layer byconducting anodic oxidation of the aluminum layer by using anelectrolytic solution including at least any of phosphoric acid, oxalicacid, and sulfuric acid.

In accordance with aspects of the invention, the optical element moldingdie that is formed by anodic oxidation using a substrate with a negativestandard electrode potential in the oxidation reaction makes it possibleto form the desired concave-convex structure of a large area on theoptical element even on a curved surface, thereby enabling massproduction and reduction of equipment cost.

Further features of the invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the first embodiment of theoptical element molding die in accordance with aspects of the invention.

FIG. 2 is a schematic diagram illustrating the second embodiment of theoptical element molding die in accordance with the aspects of theinvention.

FIG. 3 is a conceptual diagram of an average distance between centers.

FIG. 4 is a schematic diagram illustrating the fourth embodiment of theoptical element molding die in accordance with aspects of the invention.

DESCRIPTION OF THE EMBODIMENTS

The embodiments of the invention will be described below in greaterdetail with reference to the appended drawings.

First Embodiment

The first embodiment of the mold for optical element molding inaccordance with the invention will be described with reference to FIG.1.

FIG. 1 illustrates schematically a cross section of the first embodimentof the optical element molding die in accordance with aspects of theinvention.

In FIG. 1, the reference numeral 11 stands for a substrate having a tleast a surface composed of a material with a negative standardelectrode potential, 12—a protective layer formed on the substrate 11,and 13—an anodic oxidation layer provided on the protective layer 12. Alarge number of holes 14 are formed by conducting anodic oxidation inthe anodic oxidation layer 13. The holes are opened in the directionperpendicular to the surface of the anodic oxidation layer. In FIG. 1,the substrate 11 has a flat shape, but such a shape is not limiting, andthe substrate may also have a curved shape. The shape of the entireoptical element molding die is not particularly limited and may besimilar to the die shape that is generally used. For example, a die of adesired shape such as a round shape, an angular shape, and a shapecombining these shapes can be also used. The constituent components willbe described below in greater detail.

Substrate

The substrate 11 used in the present embodiment includes Ni at least atthe substrate surface to improve processability and stability inmolding. A stainless steel including Fe, Cr, Mo, and the like togetherwith Ni, or a superhard alloy using Co as a binder can be also used.These materials have a negative standard electrode potential in theoxidation reaction. The “standard electrode potential”, as referred toherein, means a value represented by ΔV in the reaction defined by thefollowing Equation (1).Mn⁺ +n ^(e−)→M+ΔV  (1)

Here, Mn⁺is a metal ion, M is a metal atom, n is integer equal to orgreater than 1, e⁻ is an electron, and ΔV is a standard electrodepotential (V).

In the optical element molding die in accordance with the presentembodiment, the entire die is immersed in an electrolytic solution andelectrolytic treatment is conducted in order to conduct anodic oxidationtreatment of the anodic oxidation layer. In a case where the substratehas a negative standard electrode potential, anodic electrolysis in theelectrolytic solution proceeds not only at the anodic oxidation layer,but also at the substrate itself, and a reversed reaction of Equation(1) proceeds. As a result, the metal is eluted as metal ions in theelectrolytic solution. Dust present on the surface of the substrate 11is difficult to remove entirely. Anodic oxidation of Al proceeds byionization of Al and oxidation thereof accompanied by dissolution, butwhen dust is present, dissolution also proceeds from the Al side surfaceof the boundary of the dust and Al. As a result, where such dust hasadhered to the substrate 11, the electrolytic solution penetrates to thesubstrate 11 through the anodic oxidation layer 13 and therefore thesubstrate 11 undergoes anodic electrolysis. The resultant defectsinclude swelling caused by generation of oxygen and spots caused bydissolution of the substrate 11. In the present embodiment, theaforementioned problems are resolved by providing the below-describedprotective layer 12 between the substrate 11 and the anodic oxidationlayer 13.

Protective Layer

The protective layer 12 used in the present embodiment includes amaterial with a positive standard electrode potential. In the materialwith a positive standard electrode potential, a reverse reaction ofEquation 1 hardly occurs in the electrolytic solution duringelectrolytic treatment. Therefore, the protective layer 12 is lesslikely to elute in the electrolytic solution. This is because, when avoltage is increased as in the case of anodic oxidation, dissolution ofelectrolytic components contained in water or a conversion liquid occursbefore the reverse reaction of Equation (1), thereby preventingdissolution and functioning as a protective layer.

A material with a positive standard electrode potential can be selectedfrom the group consisting of Ir, Au, Pt, Ru, Pd, Rh, Re, Ag, Ti, and Cu.An alloy composed of these metal elements may be also used. It ispreferred that the protective layer 12 include at least one metalelement selected from the aforementioned materials.

A layer that has a standard electrode potential higher than that of theelectrolytic solution, but is not from a metal may be also used.Examples of such suitable materials include SiO₂, Tio₂, Al₂O₃, SiN, TiN,and organic polymers.

Among the above-described materials, Au, Cu, Pt, SiO₂, and TiN arepreferably used from the standpoint of durability and easiness of filmformation when used as a die.

Anodic Oxidation Layer

The anodic oxidation layer of the present embodiment includes aluminumand has a plurality of holes opened in the direction perpendicular tothe surface of the anodic oxidation layer. These holes are formed duringanodic oxidation of the aluminum.

The anodic oxidation as referred to herein means the followingtechnique. Thus, aluminum is immersed as a positive electrode into anelectrolytic solution composed of sulfuric acid, oxalic acid, orphosphoric acid. The aluminum is oxidized by connecting a direct currentpower source between the positive electrode and a negative electrodethat is also immersed in the electrolytic solution and passing anelectric current between the electrodes, and holes of a submicron orderare formed anisotropically in the direction perpendicular to the surfaceof the anodic oxidation layer. The aluminum anodic oxidation layer isthus formed.

It is known that the pitch and depth of the holes can be controlled byappropriately selecting conditions such as voltage, temperature, andconcentration. The pitch of the holes is the distance between the centerof a hole to the center of an adjacent hole. The hole depth is thedistance from the surface of the anodic oxidation layer to the bottomportion of the hole. By appropriately selecting the conditions of theanodic oxidation process, it is possible to form a porous alumina layerhaving holes over the entire surface of the Al layer formed on thesubstrate, thereby forming a concave-convex structure rapidly andinexpensively.

In anodic oxidation, it is preferred that sulfuric acid solution be usedat a low voltage (equal to and lower than about 30 V), phosphoric acidsolution—at a high voltage (equal to or higher than 60 V), and oxalicacid solution—at an intermediate voltage. Further, the holes formed byanodic oxidation are known to have a structure with a periodicarrangement of about 2.5 times the voltage. Therefore, the period may beadjusted by using different solutions. The hole depth is proportional tothe voltage application time under various voltage and temperatureconditions. Therefore, the conditions may be appropriately selectedaccording to the desired fine hole shape. However, the hole diameterafter anodic oxidation is uniquely determined by the conditions.

A method for manufacturing the above-described optical element moldingdie of the first embodiment will be described below.

First, a substrate cut to a desired shape and surface accuracy isprepared. As for the substrate material, from the standpoint ofprocessability and stability in molding, it is preferred that Ni becontained in at least the substrate surface. A stainless steel includingFe, Cr, Mo, and the like together with Ni, or a superhard alloy using Coas a binder can be also used. The cutting may be performed directly onthe substrate. For example, it is possible to plate a SUS base with Ni—Pand then cut the plated layer.

Stains and dust present on the substrate surface are then removed. Thestains and dust can be removed by conducting degreasing or electrolyticcleaning.

A protective layer is then formed on the substrate. For example, a goldplating layer is formed by electroplating. The protective layer materialis not limited to Au and can be selected from the group consisting ofIr, Pt, Ru, Pd, Rh, Re, Ag, Ti, and Cu. Further, alloys composed ofthese metal elements may be also used. It is preferred that theprotective layer include at least one metal element selected from theaforementioned materials. A layer that has a standard electrodepotential higher than that of the electrolytic solution, but is not froma metal may be also used. Examples of such suitable materials includeSiO₂, TiO₂, Al₂O₃, SiN, TiN, and organic polymers. As a method forforming the protective layer with these materials, a method suitable forthese materials may be optionally selected from a plating method, a dipcoating method, a spin coating method which are a wet method; and avacuum deposition method and a sputtering method which are a dry method.In view of durability of using as the die and easiness of film formationamong the above materials, a material selected from at least one of Au,Cu, Pt, SiO₂ and TiN may be used as the protective layer material.

Then, for example, Ti is provided as a bonding layer by sputtering onthe protective layer and an aluminum layer is formed thereupon, therebymaking it possible to obtain a die covered with aluminum. A positiveelectrode is then attached to part of the surface other than that wherea concave-convex structure will be formed, the substrate is covered witha masking tape so as to expose only the surface where the concave-convexstructure will be formed, and the other surface is insulated andwaterproofed by this cover. The die is then immersed together with thenegative electrode, for example, in a 5 wt. % aqueous solution ofphosphoric acid adjusted to a temperature of 10° C. Holes can thereafterbe opened in the direction perpendicular to the surface of the anodicoxidation layer by applying a voltage of 120 V and passing an electriccurrent till the current amount becomes sufficiently small. A opticalelement molding die can thus be obtained that has formed thereon thedesired fine structure that is free from swelling and spots at thesurface of the anodic oxidation layer.

Second Embodiment

The second embodiment of the optical element molding die in accordancewith aspects of the invention will be described below.

In the present embodiment, the diameter of a plurality of holes formedduring anodic oxidation of aluminum is enlarged by etching in the anodicoxidation layer. The optical element molding die that has theabove-described structure will be explained with reference to FIG. 2.The first embodiment and second embodiment have many common features,the explanation of these features will be omitted, and only featuresthat are different from those of the first embodiment will be explained.

FIG. 2 is an enlarged schematic drawing illustrating the cross-sectionof a plurality of protrusions at a substrate having the concave-convexstructure of the present embodiment. In the present embodiment,similarly to the first embodiment, an anodic oxidation layer 23 isformed on a protective layer 22 that has been provided on a substrate21. The diameter of a very large number of holes that are formed byanodic oxidation at the anodic oxidation layer 23 is isotropicallyenlarged by etching.

The diameter of holes obtained by anodic oxidation is isotropicallyenlarged by etching conducted, for example, by immersion in phosphoricacid or the like. The diameter of holes is proportional to immersiontime in phosphoric acid at all temperature conditions and concentrationconditions. Therefore, the conditions may be appropriately selectedaccording to the desired fine shape.

A method for manufacturing the optical element molding die according tothe second embodiment will be explained below.

A die covered with aluminum is prepared by a method similar to that ofthe first embodiment. Then, similarly to the first embodiment, apositive electrode is attached to part of the surface other than thatwhere a concave-convex structure will be formed, the substrate iscovered with a masking tape so as to expose only the surface where theconcave-convex structure will be formed, and the other surface isinsulated and waterproofed by this cover. The substrate is then immersedtogether with the negative electrode, for example, in a 5 wt. % aqueoussolution of phosphoric acid adjusted to a temperature of 10° C. Anoptical element molding die that has formed thereon the desired fineconcave-convex structure having holes in the direction perpendicular tothe surface can then be obtained by applying a voltage of 120 V andpassing an electric current till the current amount becomes sufficientlysmall. The anodically oxidized die that has thus been obtained isfurther etched by immersing, for example, in a 5 wt. % aqueous solutionof phosphoric acid at room temperature, thereby making it possible toobtain an optical element molding die of the present embodiment that hasformed thereon the desired fine concave-convex structure having holes inthe direction perpendicular to the surface of the anodic oxidationlayer.

Third Embodiment

In the present embodiment, an optical element will be explained that ismolded by a molding process using an optical element molding diefabricated by a method similar to that of the second embodiment, thisoptical element having a plurality of concave-convex structures obtainedby transferring the surface of the optical element molding die. By usingthe optical element molding die that is produced by a method possible toinhibit the occurrence of pinholes. Therefore, the concave-convexstructure can be produced according to the designed values at the die,thereby making it possible to obtain an optical element according to thedesigned values.

Generally, when two substances with different refractive indexes and apitch shorter than a wavelength are mixed, the refractive index n₁₂ inthe mixing region can be represented with Equation (2) below by therefractive indexes (n₁, n₂) of the two substances and volume (ff₁, ff₂)occupied by each substance per unit volume.n ₁₂ =ff ₁ ×n ₁ +ff ₂ ×n ₂  (2)

When only the two substances are present in the mixing region,ff ₁ +ff ₂=1  (3)

In a case where light falls perpendicularly from the substance 1 ontothe substance 2, or from the substance 2 onto the substance 1, thehighest antireflective effect is obtained when the refractive index n₁₂of the mixing region is[Formula 1]n ₁₂=√{square root over (n ₁ ×n ₂)}  (4)

For example, when a hole is filled with air, where the refractive indexof the substance constituting the hole wall is denoted by n, thefraction ff per unit volume of the hole at which the highestantireflective effect is obtained is represented by Equation (5) below.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack & \; \\{{ff} = \frac{n - \sqrt{n}}{n - 1}} & (5)\end{matrix}$

In a case where the optical surface where the hole is formed is theoutermost surface that is in contact with air and the refractive indexof the material constituting the hole wall is 1.56, in order to obtainthe highest antireflective effect with respect to the perpendicularincidence, it is especially preferred, as shown by Equation (5), thatthe volume ratio occupied by holes be about 56%. Further, the optimumvalue of the volume ratio can be appropriately set not only by therefractive index of the material constituting the hole wall, but also bythe light incidence angle and polarization.

Where the distance between the centers of adjacent holes, from among theplurality of holes in the present embodiment, is denoted by and thewavelength used is denoted by λ, the settings are made to satisfy thefollowing condition:

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack & \; \\{p < \frac{\lambda}{\left( {{n_{1}\sin\;\theta} + n_{2}} \right)}} & (6)\end{matrix}$

The conditional formula (6) establishes the upper limit for the distancebetween the centers of the adjacent holes. Where the upper limit ofconditional formula (6) is exceeded, the resultant distance between theadjacent holes is undesirable because zero-order diffraction light isgenerated and therefore a uniformly excellent antireflectivecharacteristic is difficult to demonstrate over the entire opticalsurface. The lower limit is functionally not restricted and may be madeas small as possible, provided that volume ratio of the aforementionedhole and air is adequate.

Where the wavelength used is denoted by and the opening ratio is denotedby f, the hole depth in the optical element is set to satisfy thefollowing condition:(n1·f+n2·(1−f))·d=λ/4  (7)

In a single-layer antireflective film, it is most effective to set theoptical film thickness and so thatn·d=λ/4  (8)

where n is a refractive index of the thin film, d—a geometric filmthickness, and μ—a designed reference wavelength. In a case where therefractive index of the mixed region is uniform in the thicknessdirection, the reflection prevention can be said to be similar to thatof the single-layer film.

Here, the opening ratio means an area ratio of holes to the treatmentarea. The opening ratio can be found by binary image processing in apredetermined image region such as that of electron microscope.

Thus, by enlarging the hole diameter, it is possible to realize auniform antireflection characteristic over the entire optical surface.With the optical element molding die of the present embodiment, theoccurrence of pinholes can be inhibited. Therefore, a concave-convexstructure can be produced at the die according to the designed values.As a result, an optical element can be obtained that has a uniformantireflective characteristic over the entire optical surface.

A method for manufacturing the optical element having theabove-described concave-convex structure will be described below.

The optical element molding die that has been obtained by a methodsimilar to that of the first embodiment is observed under a scanningelectron microscope. As shown in FIG. 3, a central coordinate position31 of a hole is found by image processing, and the average value of adistance 33 between the centers with the closest six holes 32 is found.The die is then immersed in a 5 wt. % aqueous solution of phosphoricacid at normal temperature, the holes are expanded by gradualdissolution till the desired opening ratio is obtained, and the desiredfine concave-convex structure is formed.

A spacer is then provided on the die produced in the above-describedmanner to obtain a predetermined thickness and an ultraviolet curableresin is dropped. A quartz substrate subjected to a coupling processingis slowly brought into contact with the ultraviolet curable resin, pressbonded thereto, and spread to prevent the penetration of air bubbles,thereby filling the space between the quartz substrate and the diehaving the concave-convex structure of the first embodiment. Curing isthen conducted by ultraviolet irradiation from the glass platedirection. The cured product is peeled off from the substrate, and anoptical element having the concave-convex structure is obtained.

The optical element molded in the above-described manner can beadvantageously applied to image pick-up devices such as cameras andvideo cameras and projection devices such as liquid crystal projectors,displays, and optical scanning devices of electrophotographicapparatuses.

Fourth Embodiment

The fourth embodiment of the optical element molding die in accordancewith aspects of the invention will be described below.

The optical element molding die in accordance with the presentembodiment has a tapered concave-convex structure. The presentembodiment, first embodiment, and second embodiment have many commonfeatures, the explanation of these features will be omitted, and onlyfeatures that are different from those of the first embodiment andsecond embodiment will be explained.

FIG. 4 is a schematic drawing illustrating an enlarged cross-sectionalview of the optical element molding die that has a taperedconcave-convex structure of the present embodiment. In the presentembodiment, an anodic oxidation layer 43 is formed on a protective layer42 formed on a substrate 41 in the same manner as in the first andsecond embodiments. A plurality of tapered holes 44 (concave portions)are provided at the surface of the optical element molding die. Theplurality of holes 44 are formed independently from each other in thenormal direction to the surface of the optical element molding die. Thehole diameter decreases in the depthwise direction (direction toward thebottom of the holes) from the surface of the optical element moldingdie.

In a case where the size of the concave-convex structure in thetransverse direction is less than the light wavelength, the effectiverefractive index n_(e) to a certain height can be found by using thefollowing relationship from the Lorentz-Lorenz equation.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack & \; \\{\frac{n_{e}^{2} - 1}{n_{e}^{2} + 2} = {f_{1}\frac{n_{1}^{2} - 1}{n_{1}^{2} + 2}}} & (9)\end{matrix}$

where n₁ stands for a refractive index of the material forming theconcave-convex structure and f₁ is a space occupation ratio at thisheight.

Therefore, in a case where the concave-convex structure has a taperedshape and the space occupation ratio of the structure changes graduallyfrom the air (space medium) toward the substrate (incidence medium), theeffective refractive index n_(e) also changes gradually.

In the case where the refractive index changes gradually due to thetapered shape, the reflected light corresponding to the microvariationamount is generated at each height according to the Fresnel equation.Further, the reflected light beams generated at each height interfere,and in a case where the height of the concave-convex structure is equalto or greater than a predetermined value, these lights are canceled bythe interference and the reflected light is attenuated. Therefore aconfiguration of a certain height that has a tapered shape in the formof conical or tapered conical protrusions makes it possible to reducethe reflectance by comparison with that in the case of a columnar shape.

A method in which anodic oxidation and enlargement of hole diameter arerepeated in multiple stages can be used to manufacture theabove-described die of a tapered shape. First, holes are obtained byconducting anodic oxidation for a given time. Then, the die is immersedinto an aqueous solution of phosphoric acid and the hole diameter isenlarged. The die thus obtained and having the enlarged hole diameter issubjected to a second anodic oxidation treatment under conditionsidentical to those of the initial treatment. As a result, a hole isfurther formed in the depth direction inside the initially formed holethat has enlarged diameter. Immersion into an aqueous solution ofphosphoric acid is then conducted under conditions identical to those ofthe initial treatment. As a result, the diameter of the hole portionthat has been initially formed is further enlarged, and the diameter ofthe hole obtained in the second anodic oxidation is also enlarged. Theseoperations make it possible to obtain a concave-convex structureprovided with concave portions that have a two-stage diameter thatdecreases in the thickness direction. By repeating the anodic oxidationtreatment and immersion into aqueous solution of phosphoric acid in theabove-described manner, it is possible to obtain a concave-convexstructure having concave portions in the form of tapered holes in whichthe diameter decreases in multiple steps in the depth direction(direction toward the bottom of the hole). In the explanation above, theconditions of repeated operation are the same for the sake ofsimplicity, but they may be also different. Furthermore, the anodicoxidation conditions, etching conditions in the aqueous solution ofphosphoric acid, and the number of times these operations are repeatedmay be appropriately selected according to the desired shape of the fineconcave-convex structure.

This optical element molding die is used to obtain an optical elementhaving a concave-convex structure with tapered convex portions obtainedby transferring the shape of the die onto the element surface. Theconvex-concave structure shaped at the die may be transferred by moldingsuch as injection, replica, pressing, or pouring, but injection andpress molding are especially preferred because they enable efficientmolding together with the substrate.

A method for manufacturing the above-described optical element moldingdie and a method for manufacturing an optical element using the opticalelement molding die according to the fourth embodiment will be describedbelow.

First, for example, a Ni—P plating layer is formed to a thickness of 100μm on a SUS base and a substrate cut to the desired shape and surfaceaccuracy is prepared. The substrate is degreased and then stains anddust present on the substrate surface are removed by ultrasoniccleaning. After drying, for example, a SiO_(x) film is formed as aprotective layer. Then, for example, a Ti layer with a thickness of 50nm is provided as an adhesive layer on the protective layer, whilemaintaining the vacuum state, and then an aluminum layer is uniformlyformed, for example, by deposition on the adhesive layer to obtain a diecovered with aluminum. A positive electrode is then attached to part ofthe surface other than that where a concave-convex structure is wishedto be formed, masking is conducted, and an aluminum-covered die isobtained that is insulated and waterproofed by covering the area outsidethis surface. The die is then immersed together with the negativeelectrode, for example, in a 5 wt. % aqueous solution of phosphoric acidadjusted to a temperature of 10° C. A voltage of, for example, 120 V isthen applied from a direct current power source and a current is passedfor 3 min 30 sec. The hole diameter is then expanded by etching, forexample, by immersing for 45 min in a 5 wt. % aqueous solution ofphosphoric acid at room temperature and dissolving. The voltageapplication and current passing procedure and the etching procedure arerepeated a plurality of times to enlarge the hole diameter and obtainthe tapered holes. The optical element molding die that has beenproduced by the above-described procedure is then used for injectionmolding a resin or the like with an injection molding apparatus or thelike to obtain an optical element having a concave-convex structure.

The optical element manufactured in the above-described manner can beadvantageously applied to image pick-up devices such as cameras andvideo cameras and projection devices such as liquid crystal projectors,displays, and optical scanning devices of electrophotographicapparatuses. The invention will be described below in greater detail byexamples thereof. However, the invention is not limited to the examples.

EXAMPLE 1

First, a Ni—P plating layer with a thickness of 100 μm was formed on aSUS base and the surface of the plated layer was cut to prepare asubstrate processed to the desired shape and surface accuracy. Thesubstrate had a diameter of 30 mm and a thickness of 10 mm. Thesubstrate was degreased and then stains and dust present on thesubstrate surface were removed by electrolytic cleaning. A gold platinglayer with a thickness of 300 nm was then deposited as a protectivelayer by electroplating. A protective layer could thus be produced,while inhibiting the adhesion of stains and dust after cleaning, bycontinuously conducting the series of operations in a wet process.

A Ti layer with a thickness of 50 nm was provided as an adhesive layerby sputtering on the protective layer, an aluminum layer was uniformlyformed to a thickness of 300 nm on the titanium layer, and a die coveredwith aluminum was obtained. A positive electrode was then attached topart of the structure will be formed, the substrate was covered with amasking tape so as to expose only the surface where the concave-convexstructure will be formed, and a state was obtained in which the surfaceoutside this surface was insulated and waterproofed. The die was thenimmersed together with the negative electrode in a 5 wt. % aqueoussolution of phosphoric acid adjusted to a temperature of 10° C. Avoltage of 120 V was then applied from a direct current power source anda current was passed till the current amount became sufficiently small,thereby obtaining a optical element molding die that had holesperpendicular to the surface. The desired fine concave-convex structurewas formed on the surface of the optical element molding die. On thesurface of the optical element molding die, generation of swelling andspots due to dissolution of the Ni layer was not observed.

COMPARATIVE EXAMPLE 1

A conventional optical element molding die was then manufactured forcomparison. Before a protective layer was formed, a substrate wasprepared by a method similar to that of Example 1. The substrate wascleaned and dried by a method similar to that of Example 1, an Al filmwas then formed on the substrate by a method similar to that of Example1, and an anodic oxidation layer was formed. Swelling and spots wherethe Ni layer dissolved appeared on the surface of the obtained opticalelement molding die (without protective layer).

EXAMPLE 2

A Ni—P plating layer with a thickness of 100 μm was formed on a SUS baseand a substrate cut to the desired shape and surface accuracy wasprepared. Stains and dust present on the substrate surface were removedand a copper plating layer with a thickness of 300 μm was then depositedas a protective layer by electroplating. A protective layer could thusbe produced, while inhibiting the adhesion of stains and dust aftercleaning, by continuously conducting the series of operations in a wetprocess.

A Ti layer with a thickness of 50 nm was provided as an adhesive layerby sputtering on the protective layer, an aluminum layer was uniformlyformed to a thickness of 300 nm on the titanium layer, and a die coveredwith aluminum was obtained. A positive electrode was then attached topart of the surface other than that where a concave-convex structurewill be formed, the substrate was covered with a masking tape so as toexpose only the surface where the concave-convex structure will beformed, and a state was obtained in which the surface outside thissurface was insulated and waterproofed. The die was then immersedtogether with the negative electrode in a 5 wt. % aqueous solution ofphosphoric acid adjusted to a temperature of 10° C. A voltage of 120 Vwas then applied from a direct current power source and a current waspassed till the current amount became sufficiently small. The anodicallyoxidized die obtained in the above-described manner was etched byimmersing for 45 min in a 5 wt. % aqueous solution of phosphoric acid atroom temperature, thereby making it possible to obtain an opticalelement molding die that had the desired fine concave-convex structurehaving holes in the direction perpendicular to the surface. On thesurface of the optical element molding die, generation of swelling andspots due to dissolution of the Ni layer was not observed.

EXAMPLE 3

A Ni—P plating layer with a thickness of 100 μm was formed on a SUS baseand a substrate machined to the desired shape and surface accuracy wasprepared. The die was degreased and then stains and dust present on thedie surface were removed by ultrasonic cleaning. After drying, a SiO_(x)film with a thickness of 1 μm was formed as a protective layer with asputtering device. Then, a Ti layer with a thickness of 50 nm wasprovided as an adhesive layer on the SiO_(x), and then an aluminum layerwas uniformly formed to a thickness of 100 nm by deposition on theadhesive layer to obtain a substrate covered with aluminum. A positiveelectrode was then attached to part of the surface other than that wherea concave-convex structure was wished to be formed, masking wasconducted, and an aluminum-covered substrate was obtained that wasinsulated and waterproofed by covering the area outside this surface.The substrate was then immersed together with the negative electrode ina 5 wt. % aqueous solution of phosphoric acid adjusted to a temperatureof 10° C. A voltage of 120 V was then applied from a direct currentpower source and a current was passed till the current amount becamesufficiently small, thereby obtaining a substrate having holesperpendicular to the surface. The substrate thus obtained was observedunder a scanning electron microscope and very large number of holes wereconfirmed to have been formed on the substrate surface. As shown in FIG.3, a central coordinate position 31 of a hole was found by imageprocessing, and the average value of a distance 33 between the centerswith the closest six holes 32 was found. The average value of thedistance between the centers of the adjacent holes, that is, a pitch,was about 300 nm. The substrate was then immersed in a 5 wt. % aqueoussolution of phosphoric acid at normal temperature, the holes wereexpanded by gradual dissolution and a substrate having the desiredindependent protrusions was obtained. The opening ratio in this case was75%. The desired fine concave-convex structure was formed on the opticalelement molding die thus obtained. On the surface of the optical elementmolding die, generation of swelling and spots due to dissolution of theNi layer was not observed.

A spacer of 50 μm was then provided and an ultraviolet curable resin(RC-0001: manufactured by Dainippon Inks and Chemical Co., Ltd.) wasdropped. A glass substrate (BK7) subjected to a coupling processing wasslowly brought into contact with the ultraviolet curable resin, pressbonded thereto, and spread to prevent the penetration of air bubbles,thereby filling the space between the glass substrate and the die havingthe concave-convex structure of the first embodiment. Curing was thenconducted by irradiation for 750 sec at 40 mW with ultraviolet with acentral wavelength of 365 nm from the glass plate direction. The curedproduct was peeled off from the optical element molding die, and anoptical element having the desired fine concave-convex structure wasobtained.

COMPARATIVE EXAMPLE 2

A optical element molding die having no fine concave-convex structurewhich was formed before the protective layer was formed by the methodaccording to Example 3, was formed. An optical element was molded by amethod identical to that of Example 3 by using this optical elementmolding die. A surface-reflectance of the optical element obtained inComparative Example 2 and Example 3 was measured. Table 1 below showsthe results obtained by measuring the reflectance of the opticalelements at an incidence angle of 5° by a spectrophotometer. Thereflectance of the optical element having a fine concave-convexstructure was found to be lower by about 3% than that of the opticalelement having no fine concave-convex structure. The cross-section ofthe optical element thus obtained was observed under a scanning electronmicroscope and a large number of protrusions were confirmed to bepresent perpendicular to the substrate surface. Also, optical elementhaving a good appearance that no swelling and spots exist could beobtained in Example 3 and Comparative Example 2.

TABLE 1 Results Obtained in Measuring the Reflectance of OpticalElements 450 nm 500 nm 550 nm Fine structure 1.4% 1.2% 1.1% is presentFine structure 4.4% 4.4% 4.3% is absent

COMPARATIVE EXAMPLE 3

A conventional optical element molding die was then manufactured forcomparison. The substrate formed before the protective layer was formedby the method according to Example 3, was prepared to take as theoptical element molding die of Comparative Example 3. The substrate wascleaned and dried by a method similar to that of Example 3, an Al filmwas then formed on the substrate by a method similar to that of Example3, an anodic oxidation layer was formed, and the etching was performedto obtain the conventional optical element molding die of ComparativeExample 3. On the surface of the conventional optical element moldingdie (having no protective layer), swelling and spots due to dissolutionof the Ni layer were generated.

The optical element was formed by a method identical to that of Example3 by using this optical element molding die. The reflectance of theconventional optical element of Comparative Example 3 at a portion whereswelling and spots were not generated, was found to be lower by about 3%than that of the optical element of Example 2 having no fineconcave-convex structure. However, a lowering of the reflectance is notobserved at a portion where a swelling and spots-generating part wastransferred. As a result, the optical element having unevenness in anappearance was undesirably obtained by difference of the reflectancebetween the portion and a circumference thereof. The cross-section ofthe optical element thus obtained was observed under the scanningelectron microscope and a large number of protrusions was confirmed tobe present perpendicular to the substrate surface at a normal portion.At a swelling portion, holes opened by dissolution of the Ni layer weretransferred to form convex protrusions having a size of μm-order whichwere not desired. Also, in a portion where circumferential spots weretransferred, protrusions were not formed. In Comparative Example 3 thatthe protective layer was not provided, the optical element having goodappearance and good performance could not be obtained.

EXAMPLE 4

A Ni—P plating layer with a thickness of 100 μm was formed on a SUS baseand a substrate cut to the desired shape and surface accuracy wasprepared. The substrate was degreased and then stains and dust presenton the die surface were removed by ultrasonic cleaning. After drying, aTiN_(x) film with a thickness of 1 μm was formed as a protective layerwith a sputtering device of reactive system. Then, a Ti layer with athickness of 50 nm was provided as an adhesive layer on the TiN, whilemaintaining the vacuum state, and then an aluminum layer was uniformlyformed to a thickness of 500 nm by deposition on the adhesive layer toobtain a die covered with aluminum. A positive electrode was thenattached to part of the surface other than that where a concave-convexstructure was wished to be formed, masking was conducted, and analuminum-covered die was obtained that was insulated and waterproofed bycovering the area outside this surface. The die was then immersedtogether with the negative electrode in a 5 wt. % aqueous solution ofphosphoric acid adjusted to a temperature of 10° C. A voltage of 120 Vwas then applied from a direct current power source and a current waspassed for 3 min 30 sec. The hole diameter was then expanded by etchingby immersing for 45 min in a 5 wt. % aqueous solution of phosphoric acidat room temperature and dissolving. The voltage application and currentpassing procedure was repeated four times, the etching procedure wasrepeated three times, and the hole diameter was then expanded byimmersing for 8 min in a 5 wt. % aqueous solution of phosphoric acid atnormal temperature, thereby producing tapered holes. The optical elementmolding die that has thus been obtained had formed therein the desiredfine concave-convex structure. On the surface of the optical elementmolding die, generation of swelling and spots due to dissolution of theNi layer was not observed.

The optical element molding die that has been produced by theabove-described procedure was then arranged at the incoming surface sideand outgoing surface side of an injection molding apparatus (SS180manufactured by Sumitomo Heavy Industries Ltd.), poly(methylmethacrylate) (Delpet 70NH, manufactured by Asahi Chemical Co., Ltd.)was injection molded, and an optical element having a concave-convexstructure was obtained. In this case, the die temperature was 95° C. andthe pressure during resin injection was maintained at 80 MPa.

COMPARATIVE EXAMPLE 4

An optical element that was molded in Comparative Example 2 and had nofine concave-convex structure was produced. Table 2 below shows theresults obtained by measuring the reflectance of the optical elements atan incidence angle of 5° by a spectrophotometer. the reflectance of theoptical element having a fine concave-convex structure was found to belower by about 4% than that of the optical element having no fineconcave-convex structure. Also, spots and protrusive seeding having asize of μm-order were observed in an appearance.

TABLE 2 Results Obtained in Measuring the Reflectance of OpticalElements 450 nm 500 nm 550 nm Fine structure 0.5% 0.2% 0.1% is presentFine structure 4.4% 4.4% 4.3% is absent

While the invention has been described with reference to exemplaryembodiments, it is to be understood that the invention is not limited tothe disclosed exemplary embodiments. The scope of the following claimsis to be accorded the broadest interpretation so as to encompass allsuch modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application Nos.2008-286552 filed Nov. 7, 2008, and 2009-249094 filed Oct. 29, 2009,which are hereby incorporated by reference herein in their entirety.

What is claimed is:
 1. An optical element molding die, comprising: asubstrate having Ni—P plating layer formed on a base made of stainlesssteel, the Ni—P plating layer being machined to a predetermined shapeand surface accuracy; an anodic oxidation layer comprising of aluminum(Al) provided on the Ni—P plating layer; and a protective layercontaining a material selected from a group consisting of Au, Ir, Pt,Ru, Pd, Rh, Re, Ag, Ti, Cu and Si between the Ni—P plating layer and theanodic oxidation layer.
 2. The optical element molding die according toclaim 1, wherein the anodic oxidation layer has a concave-convexstructure composed of a plurality of holes that are open in a directionperpendicular to a surface of the anodic oxidation layer.
 3. The opticalelement molding die according to claim 2, wherein the holes have atapered shape such that the hole diameter decreases toward the bottomthereof.